.alpha.-Cyanoacrylates, important compounds in the manufacture of adhesive compositions, are produced industrially by reacting a cyanoacetate with formaldehyde or a polymer of formaldehyde to obtain a crude polymeric condensation product. This crude polymeric product is then depolymerized with heat and acid to yield the monomeric .alpha.-cyanoacrylate. One of the more important cyanoacrylates is methyl a-cyanoacrylate.
The basic chemical reactions for the formation of the methyl .alpha.-cyanoacrylate can be represented by the equations: ##STR1##
The depolymerization yields the desired methyl cyanoacrylate, and methyl cyanoacetate. It is difficult to separate methyl cyanoacrylate from methyl cyanoacetate by distillation because the boiling points fall close together. These equations show that more methyl cyanoacrylate can be obtained relative to methyl cyanoacetate the higher the molecular weight of the polymer (I). Nevertheless, in the past, the production of high molecular weight polymers has been avoided.
In the usual prior art process, the initial condensation reaction between the cyanoacetate and formaldehyde is conducted in a low molecular weight volatile organic solvent that is essentially insoluble in water, such as butyl acetate, benzene, toluene, heptane, or cyclohexane. These organic solvents have been the preferred reaction medium because they act as azeotroping solvents (with the water resulting from the condensation reaction) permitting the water to be removed along with the solvent by distillation.
The problem with this process is that high molecular weight polymers, which result in a higher percentage of purity for the final product, precipitate out of these solvents. In addition, these solvents frequently contaminate the final product, are extremely volatile and flammable, thus creating environmental and safety problems in practice.
Purer .alpha.-cyanoacrylate products can be obtained, and safety and environmental problems be reduced, if the azeotroping solvent can be eliminated from the manufacturing process.